Cabinets/racks for installation of 19″ (inch) wide telecom equipment are designed to fit units of multiples of 44.45 or 50 mm in height depending on referenced standard. The 19″ is generally the maximum width of a unit. To make this possible the vertical hole pattern along the sides of the rack aperture have holes according to different standards.
There are two dominant standards for vertical hole pitches. One is based on inches, the so called ANSI-standard (American National Standards Institute). The other widely used standard is based on mm, the so called ETSI-standard (European Telecommunications Standards Institute). The ANSI-standard has its holes separated 1.25″ (31.75 mm) apart and in the ETSI-standard the holes are separated by 25 mm. The horizontal c-c distance is generally 465.1 mm for both the above standards.
Since ANSI and ETSI racks apart from pitch share about the same height and width limitations, it is desirable to design a rack module which is possible to mount no matter which of the two standards are used on the cabinet/rack.
A traditional way of solving the problem of having two standards is by having fastening brackets that are exchangeable, i.e. having one ANSI-bracket and one ETSI-bracket. A somewhat more sophisticated way of solving the problem is to have an oblong hole or groove sufficiently long to cover the standards. In the example of ANSI and ETSI the length of the hole would be (31.75 mm−25 mm)/2=3.375 mm adding at least the diameter of the bolt/screw to be used.
As large structures such as cabinets/racks are built up by a number of parts that are welded or screwed together it is not hard to imagine that it's difficult to maintain a specific width within tolerances all along the front opening when each part contributes with some kind of slack. The variation in sizes of the parts is often a result of the various production methods. For instance, a rack module might be made of a bent metal sheet or assembled from several parts which add to the variation in c-c distance of the fastening holes, especially in the horizontal direction. A way of overcoming the variations is again to make oblong holes or grooves but this time horizontally.